A known frictional propulsion device for an omni-directional vehicle comprises a main wheel including an annular core member and a plurality of driven rollers (free rollers) fitted on the annular core member such that each driven roller may be freely rotatable around the tangential line of the annular core member at the corresponding position, and a pair of drive disks provided on either side of the main wheel each including a disk member and a plurality of drive rollers (free rollers) arranged along the outer periphery of the disk member and engaging the driven rollers of the main wheel so as to be each rotatable around a rotational center line at an angle to both the lateral direction and the radial direction. See WO2008/132779A1, JP2013-237327A, JP2011-63209A and JP2013-107575A, for instance.
In the disclosed frictional propulsion devices, the drive disks are rotatably supported by the body frame of the omni-directional vehicle and are individually actuated by separate electric motors so that the vehicle can be propelled in the fore and aft, oblique and lateral directions.
In these frictional propulsion devices, the main wheel is supported solely by the drive rollers supported by the drive disks flanking the main wheel and engaging the driven rollers of the main wheel. Therefore, if the drive rollers are not firmly pushed against the main wheel, the main wheel may tilt sideways or wobble as the main wheel rolls over the road surface. This tendency is particularly significant when the diameter of the drive disks is reduced, and the drive rollers engage only those driven rollers which are situated in a lower part of the main wheel.